function lazyLoad(fn, threshold = 400) {
    let flag = true;//开关是开着的
    window.onscroll = function () {
        //内容高度
        let scrollHeight = document.documentElement.scrollHeight;
        //可视区高度
        let clientHeight = document.documentElement.clientHeight;
        //内容被卷去的顶部的距离
        let scrollTop = document.documentElement.scrollTop || document.body.scrollTop;

        //公式 
        let m = scrollHeight - scrollTop - clientHeight;

        if (m <= threshold) {
            //快到底了 如何只回调一次
            if (flag) {
                flag = false; //开闭开关
                fn()
            }
        } else {
            flag = true;
        }
    }
}